Exchange and transport flashcards

Exchange and transport covers how organisms obtain substances they need and remove waste products efficiently. Students must understand why large, multicellular organisms require specialised exchange surfaces and transport systems, whereas small organisms can rely on diffusion alone. Key exchange surfaces include the alveoli in the lungs and the villi of the small intestine, both adapted to maximise the rate of diffusion. The transport of substances in mammals involves the circulatory system, including the structure and function of the heart, blood vessels and blood. Students must also understand how haemoglobin loads and unloads oxygen, the significance of the oxygen dissociation curve, and how carbon dioxide is transported. In plants, the xylem and phloem carry water, mineral ions and assimilates, and students should understand the mechanisms of transpiration, the cohesion-tension theory and translocation via mass flow.

1. Card 1

   Why do large multicellular organisms need specialised exchange surfaces?

   Their surface area to volume ratio is too small for diffusion alone to supply all cells quickly enough, so dedicated structures increase exchange efficiency.

2. Card 2

   List three features that make alveoli efficient gas exchange surfaces.

   Large surface area, thin walls (one cell thick), and a rich blood supply maintaining a steep concentration gradient for oxygen and carbon dioxide.

3. Card 3

   What is Fick's law and how does it relate to exchange surfaces?

   Fick's law states rate of diffusion is proportional to (surface area x concentration difference) divided by thickness of the exchange surface.

4. Card 4

   How is the small intestine adapted for efficient absorption of digested food?

   Finger-like villi and microscopic microvilli greatly increase surface area; a rich capillary network maintains steep concentration gradients for absorption.

5. Card 5

   Describe the sequence of events in one cardiac cycle.

   Atrial systole pushes blood into ventricles; ventricular systole pumps blood to lungs and body; diastole allows the heart to refill with blood.

6. Card 6

   How does the structure of an artery differ from a vein?

   Arteries have thicker walls with more elastic tissue and smooth muscle to withstand high pressure; veins have thinner walls and valves to prevent backflow.

7. Card 7

   What is the role of haemoglobin in oxygen transport?

   Haemoglobin is a protein in red blood cells that reversibly binds up to four oxygen molecules, forming oxyhaemoglobin in regions of high partial pressure of oxygen.

8. Card 8

   What does the Bohr effect describe?

   The Bohr effect is the shift of the oxygen dissociation curve to the right when carbon dioxide concentration rises, causing haemoglobin to release oxygen more readily to respiring tissues.

9. Card 9

   How is most carbon dioxide transported in the blood?

   About 85% is carried as hydrogencarbonate ions (HCO3-) in the plasma, formed when CO2 reacts with water inside red blood cells catalysed by carbonic anhydrase.

10. Card 10

    Explain the cohesion-tension theory of water movement in xylem.

    Water evaporates from leaves (transpiration), creating tension that pulls a continuous water column up the xylem due to cohesion between water molecules.

11. Card 11

    What is translocation and where does it occur in plants?

    Translocation is the transport of dissolved organic solutes, mainly sucrose, through the phloem from source regions (e.g. leaves) to sink regions (e.g. roots, fruits).

12. Card 12

    What factors affect the rate of transpiration in plants?

    Higher light intensity, temperature and wind speed increase transpiration; higher humidity decreases it by reducing the water potential gradient between leaf and air.

• Students often confuse the oxygen dissociation curve shifting right (Bohr effect, meaning lower affinity) with shifting left (higher affinity), leading to incorrect conclusions about oxygen loading and unloading.
• A common error is stating that water moves up the xylem by osmosis rather than by the tension generated through transpiration acting on a cohesive water column.
• Many students incorrectly describe the atrioventricular valves as preventing backflow from arteries into the ventricles, when in fact they prevent backflow from the ventricles into the atria during ventricular systole.

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